This invention relates to linear ester carbonate copolymers that contain both carbonate groups and ester groups in the linear chain.
Polycarbonate resins are known to be tough and rigid and have moderately high softening temperatures. Of particular interest are the polycarbonates of bisphenol-A diols as described in U.S. Pat. No. 3,028,365. On the other hand, polyesters such as those described from terephthalic acid, isophthalic acid and/or 1,4-butane diol are well known as molding resins having high softening temperatures but poor impact resistance.
In the past, it has been a practice to make random linear copolymers containing ester and carbonate linkages in order to obtain polymers having heat distortion temperatures generally higher than those characteristic of polycarbonates. See, for example, U.S. Pat. Nos. 3,129,121; 3,549,570; 3,053,810; 3,030,331 and 3,220,976. Unfortunately, however, the desired increase in heat distortion in often not as high as needed for many applications. More particularly, any increase in heat distortion is achieved only by sacrificing almost all of the high impact resistance that is characteristic of polycarbonate resins.
Recently, it has been found that, by alternating or ordering the ester and carbonate linkages in the ester/carbonate copolymer molecule, improved thermal resistance is achieved without a corresponding sacrifice of physical strength. See, for example, U.S. Pat. Nos. 4,156,069; 4,105,633 and 4,278,787. Such copolymers are normally prepared in a two-step procedure wherein a dihydric phenol is reacted with a diacid chloride to form a hydroxy-terminated polyester oligomer, this oligomer is then reacted with phosgene to form the desired ordered ester/carbonate copolymer. In the preparation of such copolymer having a high ester to carbonate ratio, the polyester intermediate often precipitates from solution. When this precipitation occurs, it causes problems in polymer purification loss of mechanical and optical properties and an ester/-carbonate ratio different from that which is desired.
Therefore, it is highly desirable to provide an improved process for producing ordered ester carbonate copolymers wherein the copolymer is easily purified and exhibits the desired optical and mechanical properties at the desired ester to carbonate ratio.